The accumulation of waste polymeric materials having extremely low or zero degradation rates has become a significant societal problem. For example, the number of tires produced and disposed of in the United States is estimated to be about 300 million annually. The uses for waste tires are rather limited. While they have been added to asphalt, burned in kilns, used as shock absorbers in playgrounds, and used as absorbers of oil spills, the vast majority of waste tires remain as a problematic environmental and health concern.
Natural rubber is a polymer deriving from isoprene (2-methyl-1,3-butadiene). The molecular weight of the natural rubber varies significantly and is between 104 and 106. The Merck Index defines rubber as cis-1,4-polyisoprene with a molecular weight varying from 100,000 to one million. Both natural and synthetic rubber is usually cross-linked with sulfur, peroxides or bisphenol. The process, called vulcanization, produces a three-dimensional lattice. It improves properties of the product which becomes much stronger, more temperature sensitive, more elastic and non-sticky.
While natural rubber is soluble in chloroform, absolute ether, many fixed and volatile oils, petroleum ether, carbon disulfide, and oil of turpentine, the cross-linked product is much less so. The synthetic varieties can derive from one or more of the following: 1,3-butadiene, chloroprene (2-chloro-1,3-butadiene), and similar monomers. The relevant copolymers often derive from styrene. Styrene-butadiene-rubber, or SBR, is the largest synthetic component of tires.
An estimated 60% of all manufactured rubber is used in tires. A typical passenger tire consists of natural rubber (14%), synthetic rubber (27%), carbon black (28%), steel (14-15%), and additives (fabric, fillers, accelerators, antiozonants, etc.) (16-17%). Its average waste weight is 20 lbs. A typical waste truck tire weights 100 lbs and contains reverse proportions of synthetic and natural rubber as compared with a car tire.
Waste tires are not biodegradable, thereby creating both an environmental and health problem. They exacerbate the spread of mosquito-borne diseases in that they provide an insect breeding ground. Additionally, whole tires are difficult to store in landfills in that they are bulky and cause damage to the landfill cap or seal, as they tend to “float” their way to the top of the fill. In an attempt to prevent this migration, many landfills require that scrap tires be shredded, a process which is energy intensive and wasteful if it does not produce any useful product. Due to the cost associated with proper disposal, many tires are dumped illegally.
In response to these problems, there are a number of known methodologies utilized to recycle used rubbers, all of which have significant limitations. Tires have been both combusted to produce energy and pyrolized to produce fuels. Pyrolysis is generally defined as thermal degradation at temperatures as high as 900 degree C. in an inert atmosphere and has been favored because lower temperature attempts to use tires as a source of energy have heretofore not been economic.
The breakage of rubber's covalent bonds requires that relatively large quantities of energy be delivered to the waste rubber material. Pyrolysis usually takes place at temperatures above 530 degrees C. All of these high-energy techniques use “dry” processing, in that the two reaction products are gas and solids. Some of the gas exhibits molecular weight high enough to condense with cooling water, useful as a valuable liquid fuel, but most is lower value gas and solid fuel.
Potentially economic and useful methodologies that have been attempted to introduce energy into rubber to break the covalent bonds are direct heating, ultra-sound, and microwave energy. One such approach using relatively low temperatures employs controlled oxidation, for example, the controlled oxidation of styrene-butadiene sulfur cross-linked rubber with nitric acid.
A single car waste tire weighing about 20 pounds has about the same heating value of coke, approximately 15,000 BTU's per pound, or approximately 258,000 BTU's per tire. Therefore, if economical, environmentally sound processes can be formulated to convert waste tires and other polymeric materials into a fuel source, much of the aforementioned problems can be obviated. Accordingly, there is still a continuing need for improved methods of transforming tires and other polymeric materials into fuel. The present invention fulfills this need and further provides related advantages.